The role of cholesterol oxidation products in atherogenesis has long been a controversial topic. Peng and Taylor (1983), for example, hypothesized that cholesterol oxidation products may be responsible for an initial arterial cell injury that eventually results in atherosclerosis. On the other hand, Higley et al. (1986) disputed that hypothesis, claiming instead that oxidized cholesterol is substantially less atherogenic than purified cholesterol. The analysis of the oxidized cholesterol shown in Table 1 of Higley et al. indicated the presence of diols and epoxides (78%) as the major oxidized compounds with a minor amount of hydroperoxide (18%) identified as 7.alpha.-hydroperoxide.
Hypercholesteroloemia is widely considered to be a major risk factor for the development of atherosclerosis. The lowering of blood cholesterol levels has therefore been an important goal in the search for ways to prevent or treat atherosclerosis. Medicinal agents reported as reducing development of athersclerotic lesions typically result in the lowering of blood cholesterol levels. Beneficial agents which do not affect blood cholesterol are rare. But Bell and Schaub (1986) have reported that chlorpromazine reduced the development of such lesions in rabbits fed an atherogenic diet without lowering of blood cholesterol. Chlorpromazine may function as an inhibitor of calmodulin (Gietzen, 1986) but it is classified pharmacologically as a tranquilizer and sedative (Merck Index, 1976), and therefore is not likely to be useful as a treatment for atherosclerosis.
Tipton, et al. (1987), have reported that a mixture of cholesterol auto-oxidiation products prepared from an aged sample of cholesterol inhibits calmodulin irreverisibly in a Ca.sup.2+ -dependent reaction. The reactive material was destroyed by chemical reduction. It was concluded that the calmodulin inhibition was due to one or more cholesterol hydroperoxides. The mixture of cholesterol hydroperoxides was further purified by chromatography and used in feeding experiments with rabbits. Three diets were compared, one with added cholesterol, one with added cholesterol hydroperoxides, and the third with both the cholesterol and cholesterol hydroperoxides. The diet containing cholesterol alone was found to have caused extensive atheroma formation, while the addition of the cholesterol hydroperoxides to the cholesterol diet markedly reduced atheroma formation. The cholesterol hydroperoxides were not found to lower cholesterol concentration in blood plasma, liver or heart. In a related experiment, it was found that the chemically reduced hydroperoxides were not effective in reducing atheroma formation.
Cholesterol oxidizes readily in contact with air, and the oxidation proceeds at ambient room temperature. The oxidation products as initially formed are largely cholesterol hydroperoxides. See Smith, "Cholesterol Autooxidation" (1981, Plenum Press, New York). This reference lists the initial auto-oxidation products in Table 11, pages 238 to 239. These included 3.beta.-hydroxycholest-5-ene-7.alpha.-hydroperoxide; 3.beta.-hydroxycholest-5-ene-7.beta.-hydroperoxide; and 3.beta.-hydroxycholest-5-ene-25-hydroperoxide, which is presently preferred for use in this invention. Other hydroperoxides included 3.beta.-hydroxycholest-5-ene-17-hydroperoxides; 3.beta.-hydroxycholest-5-ene-20-hydroperoxides; 3.beta.-hydroxycholest-5-ene-22-hydroperoxides; and 3.beta.-hydroxycholest-5-ene-24-hydroperoxides.
When it is desired to accelerate the oxidation of cholesterol to hydroperoxides, a photochemical oxidation procedure can be employed as described by Schenk et al. (1958). This reference describes the preparation and isolation of 3.beta.-hydroxycholest-5-ene-7.alpha.-hydroperoxide. The 7-.beta. isomer of this compound can be synthesized and isolated as described by Teng et al. (1973). Other cholesterol hydroperoxides can be prepared as an oxidized mixture, and separated by fractionation. See van Lier et al. (1970).